Blimey, thanks for the replies everyone - being a newbie I thought I'd get an email each time someone replied to the post so have only just checked back!
In the end a friend called the support line and apparently they 'used a different cloned MAC address' to get it working. It was a bit vague but I'm guessing that before the replacement cable box was put in, the router had a cloned MAC address which is from a different PC than I was using. The router must have been reset and lost the 'working' MAC address that had been entered previously.
Oh well, we'll know for next time!
Thanks again.